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a-base is a library of basic (hence the name) classes, most notably immutable collection classes with copy-on-write operations
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package com.ajjpj.abase.collection.immutable;

import com.ajjpj.abase.collection.ACollectionHelper;
import com.ajjpj.abase.collection.AEquality;
import com.ajjpj.abase.function.AFunction1;
import com.ajjpj.abase.function.APredicate;

import java.lang.reflect.Array;
import java.util.*;


/**
 * This is an immutable linked list implementation. It provides "mutators" that return copies of the list without
 *  affecting the original ("copy on write").
 *
 * The API is based on terminology of functional languages. NIL is the empty list, head< is the list's
 *  first element, tail is the list without its first element, and cons() is the
 *  operation that prepends an element to an existing list.
 *
 * @author arno
 */
abstract public class AList extends AbstractACollection> {
    private final int size;

    protected AList(int size) {
        this.size = size;
    }

    /**
     * Returns the empty list. All calls to this method are guaranteed to return the same instance.
     */
    @SuppressWarnings("unchecked")
    public static  AList nil() {
        return (AList) Nil.INSTANCE;
    }

    /**
     * Creates an AList based on the contents of an existing java.util.Iterable, copying its contents.
     */
    public static  AList create(Iterable elements) {
        if(elements instanceof List) {
            return create((List) elements);
        }
        AList result = nil();

        for(T el: elements) {
            result = result.cons(el);
        }
        return result.reverse();
    }

    /**
     * Creates an AList based on the contents of an existing java.util.List, copying its content.
     */
    public static  AList create(List elements) {
        AList result = nil();

        for(int i=elements.size()-1; i>=0; i--) {
            result = result.cons(elements.get(i));
        }
        return result;
    }

    /**
     * Creates an AList from a given list of elements.
     */
    @SafeVarargs
    public static  AList create(T... elements) {
        return create(Arrays.asList(elements));
    }

    /**
     * Returns a read-only java.util.List  view of this AList.
     */
    public java.util.List asJavaUtilList() {
        return new JuListWrapper<>(this);
    }

    @Override protected AList createInternal(Collection elements) {
        return AList.create(elements);
    }

    @Override protected AEquality equalityForEquals() {
        return AEquality.EQUALS;
    }

    /**
     * Returns this AList's head, if any.
     */
    public abstract AOption optHead();

    /**
     * Returns the list's head, i.e. its first element. If called on the empty list, it throws a NoSuchElementException.
     */
    public abstract T head();

    /**
     * Returns the list's tail, i.e. the list without its first element. If called on the empty list, it throws a
     *  NoSuchElementException.
     */
    public abstract AList tail();

    /**
     * Returns a new AList with the new element prepended. This is the only operation to 'add' elements to an AList.
     */
    public AList cons(T el) {
        return new AHead<>(el, this);
    }

    /**
     * Returns a copy of this AList with elements in reversed order.
     */
    public AList reverse() {
        AList remaining = this;
        AList result = nil();

        while(! remaining.isEmpty()) {
            result = result.cons(remaining.head());
            remaining = remaining.tail();
        }

        return result;
    }

    public int size() {
        return size;
    }

    @Override public AList toList() {
        // override as an optimization
        return this;
    }

    @Override public  AList map (AFunction1 f) throws E {
        return create(ACollectionHelper.map(this, f));
    }

    @Override public  AList flatMap(AFunction1, E> f) throws E {
        return create(ACollectionHelper.flatMap(this, f));
    }

    @SuppressWarnings("unchecked")
    @Override public  AList flatten() {
        return (AList) create(ACollectionHelper.flatten((Iterable>) this));
    }

    /**
     * Returns a java.util.Iterator over this AList's elements, allowing ALists to be used with Java's
     *  for(...: list) syntax introduced in version 1.5.
     */
    @Override
    public Iterator iterator() {
        return new Iterator() {
            AList pos = AList.this;

            @Override
            public boolean hasNext() {
                return pos.nonEmpty();
            }

            @Override
            public T next() {
                final T result = pos.head();
                pos = pos.tail();
                return result;
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }


    static class AHead extends AList {
        private final T head;
        private final AList tail;

        AHead(T head, AList tail) {
            super(tail.size() + 1);
            this.head = head;
            this.tail = tail;
        }

        @Override public AOption optHead() {
            return AOption.some(head);
        }

        @Override public T head() {
            return head;
        }

        @Override public AList tail() {
            return tail;
        }
    }

    static class Nil extends AList